1. Field of the Invention
The present invention generally relates to distortion compensation, and more particularly, to a distortion compensation technique applied to a transmitter for transmitting quadrature modulated signals in wireless digital communication systems.
2. Description of the Related Art
In mobile communications, including IMT-2000, broadband radio services are being offered, and especially, broader-band radio transmission is being discussed for the next-generation mobile communication schemes. In general, complex baseband signals are first converted to an intermediate frequency (IF) band, and the IF signals are further converted to radio frequency (RF) signals suitable for broadband radio transmission.
Such broadband mobile communication systems require bandpass filters to have steep filter characteristics, as well as a flat characteristic over the entire passband, in order to sufficiently reduce high-frequency components generated during the frequency conversion. However, since sophisticated and high-performance devices and circuits are required in broadband radio transmission, the device scale and the manufacturing cost increase consequently. To avoid this inconvenience, direct RF modulation schemes for converting baseband signals directly to RF signals are attracting attentions.
Meanwhile, in recent years and continuing, highly efficiency digital transmission schemes are widely employed in wireless communication systems. When employing a multi-level phase modulation scheme known as one of the high efficiency transmission schemes, a technique for reducing non-linear distortion in a power amplifier and adjacent channel leakage at a transmission end is required to improve power efficiency. This technique, known as distortion compensation, is an adaptive predistortion type for a transmission amplifier.
With a distortion compensating transmission amp of an adaptive predistortion type, a portion of the output signal (quadrature modulated signal) of the transmitter is subjected to quadrature demodulation to produce a feedback signal, and the feedback signal is compared with a transmission signal (reference signal) prior to quadrature modulation. Based on the comparison result, a weighting factor for distortion compensation is updated in real time. The transmission signal (reference signal) is multiplied by the updated weighting factor in order to give an inverted characteristic to the transmission signal in advance, and then quadrature modulation and power amplification are performed on the distortion compensated transmission signal. After the quadrature modulation and power amplification, the transmission signal is finally transmitted from the transmitter. See, for example, International Patent Publication WO 03/103163.
However, with such a direct RF modulation scheme, errors are generated in both the in-phase component and the quadrature component of the transmission signal to be supplied to the quadrature modulator, due to variation in analog devices and change over time. As a result, undesirable leakage of waves is generated as imaginary components in the modulated analog transmission signals, which causes degradation of the transmission signal quality.
In addition, the transmission signal (reference signal) is delayed using a delay element in the comparison process with the feedback signal in order to make the phases of the transmissions signal and the feed back signal be consistent with each other. Even if the delay time is correctly set in the delay element, the phase of the feedback signal itself fluctuates due to clock jitter caused by thermal noise or external disturbance. In short, it is difficult for a conventional adaptive predistortion technique to guarantee stable and reliable distortion compensation, which technique is likely to generate undesirable out-of-band power radiation.
Another publication, Japanese Patent Application Laid-open (Kokai) No. 6-37831A, discloses a linear transmission circuit of a wireless digital transmission scheme having a non-linear distortion compensating circuit for a high power amplifier. In this publication, the phase difference between the reference signal and the feedback signal is measured during the rising period of a burst signal, and the demodulation phase of the feedback signal is adjusted based on the measurement result so as to allow the measurement of the phase difference to be performed with least measuring error. This technique aims to guarantee correct operation of distortion compensation.
However, due to malfunction of analog circuits, including an oscillator for a down converter, the phase difference between the reference signal and the feedback signal may not be correctly determined. In this case, distortion compensation and/or other corrections cannot be performed correctly, causing abnormal operations.
FIG. 1 is a diagram illustrating phase adjustment results performed in normal operation and abnormal operation of the oscillator for a down converter. In the normal operation, the phase is set to substantially 90 degrees to maintain orthogonality. In contrast, during malfunctioning of the oscillator, the phase varies randomly ranging from −180° to 180° even after the phase adjustment.